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Translation abstract Repair welding on bridges in serviceOn steel bridges, fatigue damage repeatedly occurs in weld seams as a result of increasing traffic loads as well as design and construction defects. To carry out the repair works, the bridge must be closed to bridge traffic during re‐welding in order to prevent the movement on the crack flanks. According to the current state of the art, welding works during traffic is not allowed, because until now there has been insufficient knowledge about achievable seam qualities and fatigue strengths for welding seams in service. The aim is to provide reliable methods that allow welding work on steel structures during traffic or cyclic loading. As a basis, measurements were carried out on a representative steel bridge in order to derive realistic gap opening parameters for welding tests under cyclic loading. The description of the crack behaviour and force redistribution during welding in service presents a particular challenge in the laboratory. To quantify the influence of different welding boundary conditions, the experimental investigations were carried out based on displacement and force control. In both limit cases, standard‐compliant weld seam qualities and fatigue strengths were realised, which demonstrate the technical possibility of welding in service under defined boundary conditions. [ABSTRACT FROM AUTHOR]

Hot cracking is a frequent and severe defect that occurs during laser additive manufacturing of superalloys. In this work, a pulsed-wave (PW) laser modulation process was employed to control the solidification microstructure and reduce the hot cracking susceptibility of laser additive manufactured cobalt-based superalloy. The effects of continuous-wave (CW) and PW laser processing modes on the dendrite morphology, element segregation, eutectic phase, and hot cracking of fabricated Co-based superalloys were investigated. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to characterize the microstructural characteristics of samples. A two-color pyrometer was used to measure the molten pool temperature variation under different laser processing modes. The results show that coarse columnar dendrites, chain-like eutectic carbides, and hot cracks were observed in the CW sample. In contrast, the fine equiaxed crystals, discrete eutectic carbides, and low-level residual stresses were obtained to avoid hot cracks, owing to the high cooling rate and the periodic melting and solidification of the molten pool under the PW laser processing mode. This work provides a new method for controlling solidification structure and hot cracking of laser additive manufactured Co-based superalloy. [ABSTRACT FROM AUTHOR]

The results of studies of hot cracks formed in cast shapes from low-alloy steel G21Mn5 are presented. Macroand microstructural analyses are carried out. Fractography studies of samples cut from castings used in the production of railway freight cars are performed. The criteria used to assess the susceptibility of alloys to hot brittleness are considered. It is shown that the orientation of dendritic crystals affects significantly the formation of hot cracks, and the direction of the dendrite growth is determined by the conditions of heat dissipation, which are influenced in their turn by the geometry of the casting. The phenomenon of coalescence under the conditions of misorientation of neighbor dendrites and its effect on the formation of hot cracks is studied. A modified RDG criterion is suggested, which allows for the influence of the geometry of the cast product on the formation of hot cracks through the angle of misorientation of neighbor crystals. [ABSTRACT FROM AUTHOR]

Hot cracking is a frequent and severe defect that occurs during laser additive manufacturing of superalloys. In this work, a pulsed-wave (PW) laser modulation process was employed to control the solidification microstructure and reduce the hot cracking susceptibility of laser additive manufactured cobalt-based superalloy. The effects of continuous-wave (CW) and PW laser processing modes on the dendrite morphology, element segregation, eutectic phase, and hot cracking of fabricated Co-based superalloys were investigated. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to characterize the microstructural characteristics of samples. A two-color pyrometer was used to measure the molten pool temperature variation under different laser processing modes. The results show that coarse columnar dendrites, chain-like eutectic carbides, and hot cracks were observed in the CW sample. In contrast, the fine equiaxed crystals, discrete eutectic carbides, and low-level residual stresses were obtained to avoid hot cracks, owing to the high cooling rate and the periodic melting and solidification of the molten pool under the PW laser processing mode. This work provides a new method for controlling solidification structure and hot cracking of laser additive manufactured Co-based superalloy.

In this paper, a numerical simulation method based on the camshaft casting process and crack expansion was proposed to study the propagation characteristics of casting hot cracks. The dynamic visualisation process of hot crack growth indicates that the formation of hot tear is dominated by opening mode cracks. And during the propagation process, the equivalent stress intensity factor at the crack front first decreases and then increases. Furthermore, the propagation characteristics of the crack under different hot tear initiation conditions were studied by this method. The results reveal that the expansion ability of a hot crack is affected by the equilibrium solidification scale, solidification sequence, and solidification path of the casting. Finally, using the microscopic morphology of the cracks, the semi-quantitative analysis of the elements illustrates that the carbon content decreases sharply at the crack formation site, while manganese and sulphur are relatively rich. [ABSTRACT FROM AUTHOR]

The present study investigated existing and potential approaches for hot cracking analysis of cast alloys and showed a promising and very applicable new specimen for high alloyed steels. The reliability of this specimen has been demonstrated by adding niobium to alloy HH (ASTM A297), or alloy 1.4837 (ISO 11973). Niobium shows a very positive impact on hot cracking behavior and can thereby reduce scrap in steel foundries. Amounts above 0.6 wt% niobium can reduce the size and number of hot cracks significantly. The reduction of hot cracking tendency was quantified by the cracking factor CFNb2. The positive effect is associated with the reduction in solidification interval length that was measured by thermal analysis and the observed "healing" of cracks in the microsections. Both effects are promoted by niobium. The promotion of the ferritic solidification seems to have no positive impact on the hot cracking behavior. [ABSTRACT FROM AUTHOR]

Östenitik paslanmaz çelikler, düşük sıcaklıklarda bile çok iyi şekillendirilebilirlik, iyi mekanik özellikler ve yüksek korozyon direnci gibi özelliklere sahiptir. Bunun sonucu, tüm paslanmaz çelikler içerisinde en fazla üretilen ve kullanılan türdür. Ancak bu çeliklerin kaynağında birçok problem söz konusudur. Bunların başında, krom karbür oluşumu sonucu ısıdan etkilenmiş bölgede korozyon direncinin düşmesi gelmektedir. Bu çalışma, gaz tungsten ark kaynağı yöntemi ile AISI 316L levhalarda kaynak hatası içermeyen bağlantıların üretilmesini amaçlamaktadır. Bu amaca yönelik, 10 mm kalınlığında AISI 316L levhalar, 2,4 mm çapında ER316L dolgu teli kullanılarak beş pasoda kaynaklanmıştır. Elde edilen kaynaklı levhanın mikroyapısal ve mekanik özelliklerini araştırmak ve karakterize etmek için çekme testi yanında detaylı optik mikroskop çalışmaları ve mikro sertlik ölçümleri gerçekleştirilmiştir. Çok pasolu kaynağın mikroyapısal değişim üzerindeki etkisi ve dolayısıyla üretilen bağlantının mekanik davranışını nasıl etkilediği de araştırılmıştır. Yapılan çalışma sonucunda, kaynak bölgesinde herhangi bir hata gözlenmemiş olup, kaynaklı bağlantının çekme mukavemeti ve uzama açısından sırası ile %104 ve %58 lik bir kaynak performansı sergilediği tespit edilmiştir. Ayrıca, ergime bölgesinde ve ısıdan etkilenen bölgede sertlik artışı gözlemlenmiştir.

Magnesium alloys play important role in weight loss because of its high specific strength, most of magnesium alloy components are casting. Magnesium alloys have high hot crack sensitivity during casting and welding. Tungsten inert gas (TIG) welding parameters have great effects on hot crack sensitivity, which is important for welding and repairing alloys. However, few studies have been conducted in this regard. In this study, cracks in cast magnesium alloy AZ91D during TIG welding are examined and confirmed to be hot cracks. Different restraint and radiating conditions caused by structural affect the initiation and morphology of the hot cracks. The effect of TIG welding process parameters on thermal crack susceptibility was studied by Houldcroft crack test. Results reveal that welding parameters would significantly affect the crack length of the Houldcroft crack test specimens. Welding current and welding speed have complex and interactive effects on crack susceptibility. The present study has good effect on the magnesium alloy welding process development. [ABSTRACT FROM AUTHOR]

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This article attempts to compile a list of recent tungsten inert gas welding advancements (TIG). TIG welding only allows for a few millimeters of penetration depth. This limitation of TIG welding can be overcome by a new advanced TIG technique called activated tungsten inert gas welding (ATIG). This new approach of TIG welding allows up to 8 cm to 12 cm of penetration depth. The demand for activated tungsten inert gas welding (ATIG) has risen in the past few years, and it has been rooted as a viable alternative to subtractive manufacturing. According to research, ATIG weldments have mechanical qualities that are comparable to other more expensive welding techniques. ATIG offers significant cost savings and a higher production rate when compared to other welding techniques. ATIG, on the other hand, has a number of drawbacks, including high residual slag, poor mechanical properties, and low corrosion resistance. As a result, more research is needed into ATIG weld parameter optimization, ATIG process simulation, and post-weld heat treatment to address the aforementioned issues. In light of the foregoing, the focus of this study will be on ATIG welded sample experimental research, modeling, optimization, and corrosion behavior. Furthermore, the study will consider the significant difficulties associated with metallurgical properties, such as detrimental secondary phase formation, carbon migration, hot cracking, weld distortion, and residual stresses that must be removed or reduced for the weld to be qualified. [ABSTRACT FROM AUTHOR]

Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and watercooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center.

Hot crack formation in continuously cast steel is significantly influenced by the mechanical properties of the solid shell near its solidus temperature. Herein, a new method to study the high‐temperature mechanical behavior of the solidifying steel shell is introduced. In this method, an apparatus is designed utilizing an electric cylinder that is controlled by a servomotor to apply a specified amount of strain to the solidifying steel shell at a controlled strain rate. A special mold configuration is developed to control the dendrite growth in the direction perpendicular to the applied strain and to ensure that the strain is applied in the region of controlled shell growth. Real‐time load, displacement, and temperature data are monitored by a computer‐assisted data acquisition system. The temperature profile of the casting is predicted by MAGMASOFT and compared with experimental data. The Fourier thermal analysis method is applied to calculate a solid fraction and coupled with the temperature profile to determine the solid shell thickness during the test. The maximum strength at different temperatures for a medium‐carbon steel is determined and compared with that from the submerged split‐chill tensile test and hot tensile tests. [ABSTRACT FROM AUTHOR]

Grain refinement and the avoidance of columnar solidification is a great challenge in the deep penetration laser welds of thick-section steels. Further knowledge about the heat distribution and the temperature gradients in laser welds is vital for future attempts on the grain refinement of such welds. In this study, a comparative investigation was carried out for full and partial penetration hybrid laser welding of structural steel. The transient temperature distribution and temperature gradients were calculated for the experiments using a simplified three-dimensional finite element analysis. A comparative analysis was presented to investigate the influence of penetration mode on the temperature gradient in the liquid at different weld depths. The results of the numerical analysis suggested that, for given welding process parameters, full penetration welds have lower temperature gradients at the solidification front, meaning that they potentially have a higher chance of grain refinement, compared with partial penetration welds. [ABSTRACT FROM AUTHOR]

Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center. [ABSTRACT FROM AUTHOR]

Alloy 247 LC appertains to the group of precipitation-hardening materials and is characterized by good creep rupture resistance at higher temperatures. However, the weldability of Alloy 247 LC is severely limited due to its high tendency to cracking. The present paper identifies parameters for laser beam welding in vacuum with regard to hot crack reduction. Two different laser modes for differentiation of the beam intensity and the seam geometry are investigated. Therefore, crack evaluation is performed by SEM and light microscopy. Furthermore, the parameter dependent distortion is investigated. The results show the potential for welding of Alloy 247 LC with laser beam in a vacuum. [ABSTRACT FROM AUTHOR]

A new hybrid discrete-continuum cellular automata approach is proposed to simulate the process of new phase/grain nucleation and growth. The method couples classical thermomechanics and the logics of cellular automata switching. Within the framework of the hybrid discrete-continuum cellular automata method, the space occupied by the simulated specimen is represented as a cellular automaton—a set of ordered active elements. Every element imitates an immovable region of space related to a part of material being characterized by the certain numerical parameters. The proposed approach enables calculating the magnitude of the local force moments and simulating dissipation of torsion energy leading to the formation of new defect structures. To illustrate the capacity of the proposed hybrid discrete-continuum cellular automata approach, the numerical simulations of thermally activated recrystallization of pure titanium near crack faces were conducted. The 3D cellular automaton simulated the microstructure evolution of the V-notched specimen region that imitated the crack tip vicinity at high homologous temperatures. Calculation of heat expansion with simultaneous thermal stresses accumulation and microrotation initiation was incorporated in the simulations permitting thereby to evaluate the local entropy and to monitor the evolution of crystal defects from initiation to storage. Perspectives of the proposed algorithms for simulations of the mechanical behavior of materials experiencing thermally induced twining or phase transformations are discussed. [ABSTRACT FROM AUTHOR]

The paper presents results of the possibility of adapting the Althoff-Radtke test for High Chromium Cast Iron. The Althoff-Radtke test is a clump attempt used for steel. The Althoff-Radtke test has four different lengths of clamp which qualifies it as a test to quantitatively take into account different kinds of shrinkage ΔL. The length of the slot of the cracked corner and the length of each staple (50 - 350 mm) are the parameters tendency to cast cracks. Castings of white cast iron have a high tendency to hot cracking due to the large range of solidification temperatures, unfavorable kinetics parameters of shrinkage, and especially a lack of expansion before shrinkage. Shrinkage of high chromium white cast iron is similar to the shrinkage of cast steel, and is approximately 2%. Therefore it is important to test susceptibility to hot cracks. Research was carried out under industrial conditions. Four melts were performed, one of the initial chemical composition and the other three modified by different amounts of Fe-Ti, respectively, 0.25%, 0.5% and 0.75% Fe-Ti. The propensity for hot cracking was based on the observation of the dark surface in the corner of the sample. The study shows that the Althoff-Radtke test can be adapted to determine the tendency for hot cracking of high chromium cast iron. It should however be noted that the test results cannot be compared with those for other alloys.

P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are five main portions of defects larger than 200 mm3, four of which are interconnected. These initiated from continuous liquid film, and then were torn apart by excessive tensile stress within the brittle temperature range (BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies’ criterion and Katgerman’s criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.

In order to modify in-situ synthesized Mg2Si particles in Mg2Si/Mg-4Si composite, the modification effect of calcium-magnesia phosphate fertilizer on primary Mg2Si phase in Mg2Si/Mg-4Si composite was investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The results indicate that the morphology of the primary Mg2Si phase apparently changes from coarse dendrites to fine dispersive polygonal particles, and the mean size is decreased from 277 μm to 17 μm. With the addition of 4.0wt.% calcium-magnesia phosphate fertilizer as a modifier, the ultimate tensile strength and elongation of the Mg2Si/Mg-4Si composite are increased from 78.7 MPa and 2.1% to 105.2 MPa and 2.6%, as compared to those of the base composite, which is probably attributed to the formation of the phosphorous compound and the cluster of Ca compounds that acted as the heterogeneous nucleation substrates of the primary Mg2Si particles, resulting in a refined distribution of these precipitates.

The impact of rare earth element La on the microstructure and hot crack resistance of ADC12 alloy was analyzed. The additive amount of La was 0%, 0.3 wt%, 0.6 wt% and 0.9 wt%, respectively. The results showed that, with the increase of the additive amount from 0% to 0.6 wt%, the grain shape of α-Al gradually varied from developed dendritic crystal into fine dendritic crystal, equiaxed crystal and spheroidal crystal; eutectic silicon varied from needle-like or tabular shape into fine rod like shape; the hot crack force of the alloy also gradually decreased. However, when the additive amount of La reached 0.9 wt%, the excessive amount of rare earth elements was segregated within grain boundary area, forming intermetallic compounds. Therefore, the grain size of α-Al, eutectic silicon and the hot crack force of the alloy all increased. In the case that the additive amount of La reached 0.6 wt%, the best metamorphism effect and most excellent hot cracking resistance capacity of alloy were presented. The poisoning effect of rare earth element on eutectic silicon and the constitutional supercooling caused by rare earth element were the major causes for alloy modification, alloy refinement, and the main reasons for the increased hot cracking resistance. [ABSTRACT FROM AUTHOR]

High strength Al-Cu alloys are of great interest for processing using additive manufacturing for aerospace applications. In this study, a Ti-modified Al-Cu alloy was successfully processed by in - situ alloying laser powder bed fusion (LPBF). The relationship between the volumetric energy density (VED) and the densification of a Ti-modified and unmodified Al-Cu alloy was investigated. The results showed that a high-density, crack-free Al-Cu alloy can be achieved with the addition of Ti. Hot cracks occurred along the coarse columnar grains in the unmodified Al-Cu alloy due to the severe stress concentrations arising during the LPBF process. The elimination of hot cracks was attributed to the decrease of grain size and the change of grain morphology from columnar to equiaxed with the Ti addition. The tensile properties of the Ti-modified Al-Cu alloy were significantly improved with a yield strength of 200 MPa, an ultimate strength of 289 MPa, and an elongation to failure of 8 %. The strengthening mechanisms including grain refinement strengthening, solid solution strengthening and second phase strengthening were quantified and the fracture mechanism was discussed. The tribological results showed that a low wear rate of 3.03 × 10−4 mm3 N−1 m−1 was achieved in the alloy with Ti modification, while unmodified alloy showed a poor wear resistance. The study demonstrates a novel method to develop advanced Al-Cu alloy with high density, strength and tribological resistance by in - situ alloying LPBF. • Al-Cu alloy and Ti-modified Al-Cu alloy was processed by laser powder bed fusion. • The Ti-modified Al-Cu alloy with crack-free structure was obtained. • The Ti-modified specimen showed better wear resistance and mechanical properties. [ABSTRACT FROM AUTHOR]

When designing piping systems for various industrial facilities, carbon steel and stainless steel are widely being used. In order to satisfy design requirements in the piping systems, the two different materials are often welded in various cases. Therefore, for quality assurance, it is necessary to understand mechanical and metallurgical properties of dissimilar metal welds thoroughly. In this study, dissimilar metal welds of stainless and carbon steels were produced through the gas tungsten arc welding (GTAW) process. In the middle of the dissimilar weld, buttering welding and butt welding were manufactured using filler wires of ERNiCr-3 and ER316L. The chemical composition of the dissimilar metal weld was analyzed. Tensile test, bending test, and hardness test were additionally performed. The microstructures of the dissimilar metal weld were investigated to analyze the cracks found during the tensile test and the bending test. The metallographic behavior was analyzed in the vicinity of the cracks. The mechanism and cause of the cracks in the dissimilar metal weld were identified. As a result, the precipitates of complex carbide types were observed in segregation bands.

In the aircraft industry, double-sided laser beam welding is an approved method for producing skin-stringer T-joints on aircraft fuselage panels. As for the welding of new generation aluminum-lithium alloys, however, this technique is limited because of high hot cracking susceptibility and strengthening elements’ uneven distributions within weld. In the present study, a new technique of skin embedded wire double-sided laser beam welding (LBW) has been developed to fabricate T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys using eutectic alloy AA4047 filler wire. Necessary dimension parameters of the novel groove were reasonably designed for achieving crack-free welds. Comparisons were made between the new technique welded T-joint and conventional T-joint mainly on microstructure, hot crack, elements distribution features and mechanical properties within weld. Excellent crack-free microstructure, uniform distribution of silicon and superior tensile properties within weld were found in the new skin embedded wire double-sided LBW T-joints. [ABSTRACT FROM AUTHOR]

In order to modify in-situ synthesized Mg2Si particles in Mg2Si/Mg-4Si composite, the modification effect of calcium-magnesia phosphate fertilizer on primary Mg2Si phase in Mg2Si/Mg-4Si composite was investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The results indicate that the morphology of the primary Mg2Si phase apparently changes from coarse dendrites to fine dispersive polygonal particles, and the mean size is decreased from 277 µm to 17 µm. With the addition of 4.0wt.% calcium-magnesia phosphate fertilizer as a modifier, the ultimate tensile strength and elongation of the Mg2Si/Mg-4Si composite are increased from 78.7 MPa and 2.1% to 105.2 MPa and 2.6%, as compared to those of the base composite, which is probably attributed to the formation of the phosphorous compound and the cluster of Ca compounds that acted as the heterogeneous nucleation substrates of the primary Mg2Si particles, resulting in a refined distribution of these precipitates. [ABSTRACT FROM AUTHOR]

In this study, solidification cracking in pulsed laser welding of fully austenitic, AISI Type 316 stainless steel has been analysed at different energy transfer modes. The pulse parameters have been selected appropriately to obtain conduction, transition and keyhole mode welds. Conduction and transition mode welds exhibit higher susceptibility to cracking than keyhole mode welds. It is observed that both heat input and energy transfer mode affect the cooling rate and hence influence solidification cracking. Microstructures of the fusion zone have been analysed, and the cooling rate experienced by the weld is estimated from the mean cell size in the weld. It is found that the critical cooling rate below which cracking does not occur is ∼104 K s− 1. [ABSTRACT FROM AUTHOR]

The formation of undesirable coarse columnar grains and periodic hot cracks has been a long-standing issue in the additive manufacturing (AM) of unweldable high-strength aluminum alloys and their nanocomposites. Herein, the LPBF-printability of AA2024-based nanocomposites is improved by TiC-induced in-situ reinforcement and nanoparticle-enabled grain refinement during laser powder bed fusion (LPBF). The TiC-triggered in-situ reaction during LPBF overcomes the poisoning of the TiC grain refiner that occurs during conventional solidification. It not only results in fine equiaxed grains with a minimum average size of ∼1.6 μm but also suppresses hot crack formation. Powerful L1 2 -Al 3 Ti nucleants are incorporated in-situ into AA2024 through TiC nanoparticle addition during LPBF, promoting the heterogeneous nucleation of α-Al. Moreover, the growth restriction effect induced by residual TiC nanoparticles and lamellar Al 4 C 3 nanoparticles through pinning behavior along grain boundaries also contributes to grain refinement. Furthermore, these particles serve as effective reinforcement particles as well as grain refiners. The as-built AA2024-based nanocomposites tailored by nano-TiC show a maximum ultimate tensile strength (UTS) of ∼388 MPa, a yield strength (YS) of ∼332 MPa and an elongation (El) of ∼10.2%. After T6 heat treatment, the nanocomposites exhibit an outstanding UTS of ∼507 MPa, a YS of ∼456 MPa and an El of ∼6.6%. This nano-TiC-induced microstructural control method provides new insights into successful AM of hot-crack-sensitive high-strength aluminum alloys and their composites. [ABSTRACT FROM AUTHOR]

As for steel frames of buildings, the welding of single-bevel T-joints with steel backing is usually used. Recently, there have been some experimental reports concerning 25 degree groove welding by semi-automatic arc welding. However, it is not yet carried out in the actual field. The shortening of welding time leads to a reduction in emissions of greenhouse gas. Therefore, as this is an earth environment improvement, we expect this welding process in the steel frames of buildings to become attractive. In the same way as semi-automatic arc welding, the welding of the first layer is one of the biggest problems for the narrow groove welding in steel frames of buildings by welding robots. In this study, we researched actual assembling precision of T-joints, and we found the relationship between welding condition and crack occurrence. Our results demonstrated the probability of narrow groove welding of T-joints between square steel tubes and diaphragms in which robot welding was used. [ABSTRACT FROM AUTHOR]

Abstract: 6000 series (magnesium and silicon based) aluminum alloys are highly susceptible to hot cracking during laser beam welding. The occurrence of hot cracks depends on thermo-mechanical strain at the solidifying stage of the melt and in the micro-scale on the solidification structure. A new metallurgical approach to avoid these cracks by affecting the solidification process is using a multi-alloy aluminum with a silicon-rich layer. Within this paper infrared videos were used to investigate and compare the behavior of this new material with the 6xxx monolithic alloy at hot crack sensitive close-edge positions. [Copyright &y& Elsevier]

In this paper, metallographic examinations, characterising microstructural changes in the 713C superalloy subjected to remelting by GTA method, are presented. In the fusion zone, precipitation of MC or MC carbides based on chromium and molybdenum was observed. Eutectic mixtures of ( γ- gg′)-MC type with highly developed morphology were also perceived. It was found that, in the matrix areas with non-homogeneous chemical composition, the eutectic reaction γ-γ′ can occur at the temperature close to that of the precipitation of the MC carbides. The presence of silicon in the carbide phases can be conducive to lowering their solidification point by creating low-melting compound NbSi. Both in the fusion zone (FZ) and in the heat-affected zone (HAZ), the secondary precipitates of the Ni(AlTi)- γ′ phase, varying in size from 50 to 100 nm, were found. The lattice mismatch factor of the γ and γ′ particles was +0.48 % to +0.71 %, which is characteristic of the coherent precipitates of the NiAl phase enriched with titanium. No dislocations or stacking faults were observed in the microstructure of the FZ. In the HAZ, some primary undissolved γ′ precipitates, with a part of aluminium probably replaced with niobium were observed, which raised their melting point. [ABSTRACT FROM AUTHOR]

Hot crack formation during solidification was investigated during the peritectic reaction in steels. A series of in situ solidification experiments was performed using a MTS tensile testing machine combined with a mirror furnace. Sample temperature and force change were measured during the solidification process. The force measurements showed a sudden drop during the solidification of samples, which occurred around the peritectic temperature of the alloy, were accompanied by cracks or refilled cracks in the microstructure. Furthermore, the peritectic reaction types were studied theoretically and experimentally to understand their effects on the force change during solidification. The theoretical analyses showed that the volume change due to the peritectic transformation is one of the reasons for crack formation. In addition, when the peritectic reaction occurs in a diffusionless (partition less) manner in an alloy with sufficiently high primary precipitation, crack formation is more probable. [ABSTRACT FROM AUTHOR]

• The hot cracks are eliminated in the LPBF-processed ZK60. • The grain size is significantly reduced after the powders modified by Y. • The grain boundaries are strengthened by the I - phase. • The cracks in LPBF-processed ZK60 samples are attributed to the Mg 7 Zn 3. The hot cracks of laser powder bed fusion (LPBF) processed ZK60 components were eliminated by adding rare earth element Y. The cause of the cracks in the samples and the suppression mechanism were analyzed in detail. The cracks in ZK60 LPBF samples were mainly caused by the low-melting eutectic phase Mg 7 Zn 3 at the grain boundaries. The average grain size decreased from 7.2 μm to 2.0 μm after adding Y. The I-phase with excellent thermal stability strengthened the grain boundaries. The elimination of cracks was mainly attributed to the I-phase at the grain boundary and the grain refinement. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents results of microstructural observations and phase analysis of electron-beam-welded fusion zones in superalloy Inconel 713C using transmission electron microscopy. In the fusion zone, a 90% fraction of fine-grained γ′ precipitates was found, with sizes up to 30 nm. No dislocations were observed in the precipitates or at the γ–γ′ interface. Primary, undissolved inclusions of γ′ were found in the heat-affected zone (HAZ). In the HAZ, a very high concentration of dislocations was found at the γ–γ′ boundaries, as well as inside the γ′ particles and in the γ solid solution. The increased dislocation density indicates loss of coherence of that phase and the creation of a semi-coherent boundary, and is related to dissolution of the particles and intensified diffusion through the interphase γ–γ′ boundary. The lattice misfit coefficient Δa/a between the γ′ particles and γ solution in the HAZ indicates negative values from −0.20% to −0.06%. The presence of semi-coherent boundaries and the negative lattice misfit coefficient leads to dislocation locking and can result in cracking in the HAZ. [Copyright &y& Elsevier]

Abstract: The paper presents the results of metallographic examination of microstructural changes in the matrix of γ–γ′ superalloy Inconel 713C, subject to electron beam fusion. The obtained fusions simulate the phenomena that occur in the weld and the heat-affected zone during welding. Changes of γ′ phase morphology in the fusion zone, in HAZ and in the areas adjacent to grain boundaries that can affect crack initiation were determined. Changed morphology in the recrystallized γ′ zone can result from its reduced melting point due to impoverishment in niobium and titanium as well as to changed aluminium concentration or accelerated diffusion of alloying elements. The cracks found in HAZ propagate mainly through grain boundaries in the microporosity areas or, possibly, the areas left by molten carbide phases. However, no secondary phases with lower melting point or eutectics like (γ–γ′) (M x C y +Zr+Si) were found in the neighbourhood of cracks. The cracks present in the fusion zone were initiated at high temperature and propagate in the areas of interdendritic voids. The cracks were caused by the high solidification rate in the fusion zone and low thermal conductivity of the alloy. [Copyright &y& Elsevier]

• According to the characteristics of pulsed laser, the application of pulsed laser in alloy welding was summarized. • By discussing the crack sensitivity criterion, the mechanism of pulse laser welding to solve welding crack was analyzed. • The outlook for the future trends of pulsed laser welding and crack criterion was presented. There are mainly two kinds of laser used in welding of materials, continuous laser beam welding (CLBW) and pulsed laser beam welding (PLBW). The two kinds of laser welding have each welding characteristics. Compared with CLBW, the heat input of PLBW can be controlled lower, and has been widely used in the connection of thin plate materials. Since the pulse shaping can be edited to realize the preheating and slow cooling of each welding process, pulsed laser has been studied to suppress hot cracks and other defects in the field of aluminum alloy, magnesium alloy and other materials welding. This article describes the application of pulsed laser in alloy welding. The main welding problems and prevention methods were summarized. By discussing the index had been proposed to predict the sensitivity of solidification cracks, the formation and growth processes of hot cracks in pulsed laser welding was more comprehensively analyzed. Finally, the outlook for the future trends of pulsed laser welding is presented. [ABSTRACT FROM AUTHOR]

When designing piping systems for various industrial facilities, carbon steel and stainless steel are widely being used. In order to satisfy design requirements in the piping systems, the two different materials are often welded in various cases. Therefore, for quality assurance, it is necessary to understand mechanical and metallurgical properties of dissimilar metal welds thoroughly. In this study, dissimilar metal welds of stainless and carbon steels were produced through the gas tungsten arc welding (GTAW) process. In the middle of the dissimilar weld, buttering welding and butt welding were manufactured using filler wires of ERNiCr-3 and ER316L. The chemical composition of the dissimilar metal weld was analyzed. Tensile test, bending test, and hardness test were additionally performed. The microstructures of the dissimilar metal weld were investigated to analyze the cracks found during the tensile test and the bending test. The metallographic behavior was analyzed in the vicinity of the cracks. The mechanism and cause of the cracks in the dissimilar metal weld were identified. As a result, the precipitates of complex carbide types were observed in segregation bands. [ABSTRACT FROM AUTHOR]